LA JOLLA,
Calif.—In a new study led by Associate Prof. Bruce Torbett of The Scripps Research Institute
(TSRI), a team of researchers have cleared a major hurdle by solving the puzzle of how to bypass blood stem cells’ natural defenses and more
efficiently insert disease-fighting genes into those cells’ genome.

The solution to the dilemma is
rapamycin, a drug which is commonly used to slow cancer growth and prevent organ rejection. Torbett and his team discovered that the drug also enables
delivery of a therapeutic dose of genes to blood stem cells while preserving stem cell function.

This discovery is
considered a huge find toward making gene therapy a more achievable reality.

“We are really excited to have leaped over this hurdle in gene therapy,” Torbett adds. “The bottom line is, ultimately, I’d
like to see this use get to benefit patients.”

These findings, published in the online version of the
journal Blood, could lead to more effective and affordable long-term treatments for
blood cell disorders in which mutations in the DNA cause abnormal cell functions, such as in leukemia and sickle cell anemia.

Torbett and his team initially set out to test whether rapamycin, chosen for its ability to control virus entry and slow cell growth, could
improve delivery of a gene to blood stem cells.

Viruses infect the body by inserting their own genetic material
into human cells, he said. In gene therapy, however, scientists have developed “gutted” viruses, such as the human immunodeficiency virus
(HIV), to produce what are called “viral vectors” that carry therapeutic genes into cells without causing viral disease.

For a disease such as leukemia, in which mutations in the DNA cause abnormal cell function, efficiently targeting the stem
cells that produce these blood cells could be a successful approach to halting the disease and prompting the body to produce healthy blood cells, he
says.

“If you produce a genetic modification in your blood stem cells when you are five years old, these
changes are lifelong,” Torbett notes. Furthermore, the “gene-modified stem cells can develop into many types of cells that travel throughout the
body to provide therapeutic effects.”

However, because cells have adapted defense mechanisms to overcome
disease-causing viruses, engineered viral vectors can be prevented from efficiently delivering genes, he points out.

So when scientists extract blood stem cells from the body for gene therapy, HIV viral vectors are usually able to deliver genes to only 30 to 40
percent of them, Torbett says. For leukemia, leukodystrophy or genetic diseases where treatment requires a reasonable number of healthy cells coming from
stem cells, this number may be too low for therapeutic purposes.

This limitation prompted Torbett and his team,
including TSRI graduate student Cathy Wang, the first author of the study, to test whether rapamycin could improve delivery of a gene to blood stem
cells.

The researchers began by isolating stem cells from cord blood samples, Torbett said. They then exposed the
blood stem cells to rapamycin and HIV vectors engineered to deliver a gene for a green florescent protein, providing a visual marker that helped the TSRI
team track gene delivery.

“We saw a big difference in both mouse and human stem cells treated with
rapamycin, where therapeutic genes were inserted into up to 80 percent of cells,” Torbett explains. “This property had never been connected to
rapamycin before.”

The TSRI researchers also found that rapamycin can keep stem cells from differentiating
as quickly when taken out of the body for gene therapy, allowing more time to work on extracted blood stem cells, he says, adding that once these cells leave
the body, they begin to differentiate if manipulated into other types of blood cells, and lose the ability to remain as stem cells and pass on therapeutic
genes.

“We wanted to make sure the conditions we will use preserve stem cells, so if we transplant them back
into our animal models, they’ll act just like the original stem cells,” Torbett says. “We showed that in two sets of animal models, stem
cells remained and produced gene-modified cells.”

Wang stated in a news release, “Our methods could
reduce costs and the amount of preparation that goes into modifying blood stem cells using viral vector gene therapy. It would make gene therapy accessible
to a lot more patients.”

The next steps are to carry out preclinical studies using rapamycin with stem cells
in other animal models, then test whether the method is safe and effective in humans, Wang said. The team is also working to delineate the dual pathways of
rapamycin’s method of action in blood stem cells.

The abstract in the Blood journal states in part: “Transplantation of genetically modified hematopoietic stem cells (HSCs) is a promising
therapeutic strategy for genetic diseases, HIV and cancer. However, a barrier for clinical HSC gene therapy is the limited efficiency of gene delivery via
lentiviral vectors (LV) into HSCs … Collectively, rapamycin strongly augments LV transduction of HSCs in vitro and in vivo, and may prove useful for
therapeutic gene delivery.”